1. Field of the Invention
The present invention relates to a semiconductor device and to a method for manufacturing the same by selectively forming a metal thin film on a semiconductor substrate as a contact plug.
2. Description of the Related Art
As the high integration and micro miniaturization of a semiconductor device have recently been promoted, a multi-layer wiring technology becomes essential. In order to accomplish the multi-layer wiring structure, a device region of a MOS or bipolar transistor and the metal wire, or a plurality of metal wires must be interconnected through an insulation film.
In the past, in order to conduct such wiring, a desired contact hole is formed in the insulation film and a metal wire is buried in the contact hole simultaneously with the formation of an upper layer metal wire. However, recently, a diameter of the contact hole is in the order of sub-micron and an aspect ratio of the contact hole formed in the insulation layer is larger than unity. The aspect ratio is defined as a ratio of a depth to a diameter of the opening. As the aspect ratio increases, the metal wire does not fully go into the contact hole and positive connection may not be attained.
In order to avoid the above problem, a tungsten selective growth method as disclosed in JP-A-4-25159 has been proposed.
The wiring formation method of the above patent application is briefly explained. First, an insulation film is formed on a semiconductor substrate and a contact hole is formed in the insulation film. A relatively thin metal silicide film is formed on an entire surface of a semiconductor substrate as a seed for selective growth and then a photo-resist film is formed on the semi-conductor substrate, and then the photo-resist film and the metal silicide film are etched back to keep the metal silicide film only within the contact hole. After the photo-resist film remaining in the contact hole is removed, a tungsten film is selectively grown in the contact hole by using the metal silicide as the seed. Thus, a wiring layer including the tungsten film which fully fills the contact hole is formed. Since the tungsten film is hard to be formed on the insulation film, the metal silicide film is formed as the seed for the selective growth.
However, in the method of the above mentioned patent publication, since the metal silicide film remains at the opening portion of the contact hole (an upper edge of the contact hole), when the tungsten film is selectively grown in the contact hole by using the metal silicide film as the seed and the contact hole is filled with the tungsten film, the tungsten film also grows from the opening portion of the contact hole and the tungsten film spreads out of the opening portion of the contact hole. As a result, when the metal wiring is formed on the tungsten film spread out of the opening portion of the contact hole, inferior wiring shape or inter-wiring short-circuit may occur.
JP-A-5-283536 discloses a manufacturing method which prevents titanium/titanium nitride on an inner wall of a contact hole from being etched by misalignment when a conductive film formed on the contact hole is etched to form the wiring.
This method is briefly explained. After the titanium/titanium nitride is sputtered on the inner wall of the contact hole formed in an interlayer insulation film on a semiconductor substrate, tungsten is vapor-grown to fill the contact hole. Then, the surface of the interlayer insulation film is exposed by dry etching and only the titanium/titanium nitride is selectively etched to form a groove. After a silicon oxide film is vapor-grown to fill the groove, the surface of the tungsten is exposed by dry etching and a conductive film is sputtered thereon. The conductive film is dry etched by using a photo-resist as a mask to complete the wiring.
In this method, however, an area of the contact is substantially reduced by the thickness of the silicon oxide film and a higher accuracy is required for positioning when the wiring is formed on the contact hole. On the other hand, if the contact hole is enlarged accordingly, it is against the high integration of the semiconductor device. Further, a process of vapor growing the silicon oxide and a process of exposing the surface of the tungsten by the dry etching are required and it makes the manufacturing process of the semiconductor device complex.